1. Field of the Invention
The present general inventive concept relates to a fixing unit and an image forming apparatus having the fixing unit, and more particularly, to a fixing unit to form a uniform fixing nip to which heat and pressure are uniformly applied along a longitudinal direction thereof by preventing pressing rollers from having a bending deformation, and an image forming apparatus having the fixing unit.
2. Description of the Related Art
In image forming apparatuses, light is illuminated onto a uniformly charged photosensitive drum to form a predetermined latent image, the latent image formed on the photosensitive drum is developed into a visible image by supplying a toner thereto, and the developed image is transferred and fused onto a printing medium to be printed.
In a conventional fixing unit, two rollers closely contact each other, that is, a heat roller, which includes a heat source therein, and a pressing roller, which is closely positioned to come in contact with the heat roller by applying a predetermined pressure thereto, to form a fixing nip along a longitudinal direction thereof, and a toner image is fused onto a printing paper when the printing paper, on which the toner image is transferred, passes through the fixing nip. To obtain a fixing nip that is sufficient for a desired fixing quality, a size of a pressure area between the two rollers (i.e., the fixing nip) should be increased. If a diameter of the pressing roller is increased to this end, a size and weight of the fixing unit is also increased. As a result, an image forming apparatus having the fixing unit thus becomes large, and a material cost thereof thus rises. Accordingly, there are limits on increasing the diameter of the pressing roller. As an alternative method of increasing the size of the fixing nip, an elastic pressure of a spring that presses the pressing roller to the heat roller may be increased. In this case, however, the pressing roller may be bent and deformed due to the increased elastic pressure. As a result, the size of the fixing nip is decreased at a center portion of the pressing roller in a longitudinal direction thereof, and sufficient pressure cannot be applied at the center portion, which leads to a problem that toner particles cannot be sufficiently fused onto the printing paper.
In recent years, a fixing unit having a structure in which two pressing rollers having relatively smaller diameters contact the heat roller sequentially along an outer circumference of the heat roller has been developed. There have been merits in that the fixing unit is less limited in size or weight by providing small pressing rollers, and the size of the fixing nip can be sufficiently increased by providing a plurality of pressing rollers. However, the rigidity of the pressing rollers deteriorates as the sizes thereof are reduced, so the pressing rollers becomes severely bent and deformed.
FIG. 1 is a view illustrating a deformed pressing roller, and a state of bending in FIG. 1 is exaggerated for convenience. Referring to FIG. 1, an elastic force F is applied at both ends of a pressing roller 10 to press the pressing roller 10 to closely contact a heat roller 30, and the pressing roller 10 is thereby bent and deformed. More specifically, the contact between the pressing roller 10 and the heat roller 30 gradually deteriorates from one end of the pressing roller 10 to a center portion thereof in a longitudinal direction of the pressing roller 10, and a curve portion C is formed on the center portion of the pressing roller 10. In this case, heat and pressure conditions become uneven in a width direction of the printing paper (corresponding to the longitudinal direction of the pressing roller 10), which leads to poor fixing, and thus sufficient heat and pressure required for fixing is not delivered to the curve portion C formed in the center portion of the pressing roller 10. For a reference, the bending deformation of the pressing roller 10 can be quantified into a vertical distance e that is measured from one end of the pressing roller 10 to the curve portion C in the center portion thereof in the longitudinal direction of the pressing roller 10. For example, the distance e can be approximately 0.117 mm. In addition to a resulting poor printing quality, excessive elastic force applied to end portions of the heat roller 30 and the pressing roller 10 reduces a lifespan of entire fixing units, including the heat roller 30.